Manufacture of calcium sulphite filled paper



Patented Apr. 18. 1939 MANUFACTURE OF CALCIIM SULPHITE FILLED PAPER Francis G. Rawling, Piedmont, W. Va., assignor, by mesne assignments, to West Virginia Pulp & Paper Company, New York, N. Y., a corporation of Delaware No Drawing. Application December 17; 1934, Serial No. 757,888

9 Claims.

This invention relates to a method of stabilizing calcium sulphite by preventing the conversion of calcium sulphite into calcium sulphate or removing or reconverting the sulphate when formed either in the course of producing the sulphite or in utilizing it or products containing it.

It has been found that in the production of calcium sulphite by the reaction, for example, between a suspension of milk of lime and an atmosphere rich in sulphur dioxide there is a tendency to convert a portion of the sulphite into calcium sulphate. This is apparently due to oxidation of the sulphite by means of the free oxygen present in the absorber in which the reaction takes place. Inasmuch as the atmosphere containing sulphur dioxide, used in the reaction, is

most readily obtained by burning sulphur in an ample supply of air it is difiicult to eliminate the excess oxygen in this atmosphere and thus pre- 20 'vent the objectionable conversion in that way. Similarly in the use of calcium sulphite as a filler in the manufacture of paper or a variety of other products it is ordinarily difilcult to exclude oxygen in the course of mixing the sulphite with pulp or other ingredients. Accordingly there arises the same objectionable conversion of the,,su1phite into the sulphate.

Probably the most serious tendency toward conversion arises in the treatment of broke derived from paper containing calcium sulphite as a filler or a coatingsubstance. In repulping such broke in a beater considerable opportunity is afforded for intimate contact with the air and this leads to oxidation of the calcium sulphite in the manner explained.

It will be appreciated that in the use of calcium sulphite for various purposes it will be desirable, for one reason or another, to maintain its purity as far as possible and prevent its conversion into the sulphate. For example in the manufacture of paper it will be found that the presence of calcium sulphate in conjunction with sizing ingred ients such as rosin and alum leads to the production of a stable foam which. accumulates at the web of paper. This objectionable'efiect becomes cumulative due to the fact that more and more broke is developed and hence more and more foam is produced.

Now I have discovered that the conversion of calcium sulphite to the sulphate can be prevented or materially arrested or the action may even be reversed by the addition to the broke or to the suspension of calcium sulphite, or the mixture 55 from which it is being formed, of a small quantity the slices and causes foam spots and breaks in of a suitable anti-oxidant, or an agent which will tend to drive the reaction toward the production of calcium sulphite rather than the sulphate. 'For this purpose I have found sodium sulphite,

barium sulphite or sodium thiosulphate to be 5 exceptionally satisfactory. The desirable action of these substances is probably due to the fact that they retard the oxidation of the calcium sulphite or in the case of sodium sulphite it may be due to the, fact that when any calcium sulphate is formed it reacts with the sodium sulphite to reconvert the sulphate into calcium sulphite. In any event the desired result of eliminating calcium sulphate from the final product is brought about. Glucose, sugar, and tin salts, such as stannous chloride, have also been found to have the desired effect, probably by way of preventing oxidation and these may, therefore, be used advantageously in lieu of all or part of the sodium sulphite or sodium thiosulphate. In fact any desired combination of these various substances may be employed.

From another angle of approach the conversion difiiculty may be overcome by adding to the calcium sulphite, or to the material containing it or to the mixture from which it is formed, a suitable quantity of a soluble salt of an acid whose calcium salt is less soluble than calcium sulphate in water, and which is capable of reacting with calcium sulphate. Salts of this character are "sodium sulphite, sodium silicate, sodium phosphate, and the like, and also barium carbonate and barium sulphite. When these salts are present the reaction is driven in the direction away from the production of calcium sulphate and hence any of the sulphate previously formed is either reconverted to calcium sulphite or is otherwise removed. Excellent results may be obtained by the combined use of any of the substances just mentioned and sodium thiosulphate since the latter will inhibit oxidation and the further production of calcium sulphate while the former will reconvert or remove any of the sulphate previously formed.

In the use of any of the substances mentioned 45 herein nothing is added or produced by reaction which willinterfere with the use of the, calcium sulphite in the manufacture of paper. The foaming difliculties'arising from the presence of calcium sulphate will be overcome and no other deleterious effects will be produced in their place.

In the maceration and repulping of broke containing calcium sulphite it will ordinarily be de- 1 sirable to add between 1 and 2%, based upon the weight of the broke, of the substance or mixture of substances for removing or preventing formation of calcium sulphate. If one or more ofthese substances is added to the milk of lime or the like used in the production of calcium sulphite or is added to'or with the calcium sulphite being introduced into the paper stock the quantity required will vary with the particular circumstances but will ordinarily be between 2 and 10% of the weight of the lime or calcium. sulphite. It will be understood, however, that the figures specified are simply illustrative of the sorber the agentmay, for example, be introduced in a water solution or suspension either directly into the absorber or into the lime or carbonate suspension prior to introduction of the latter into the absorber. When the action'of the agent is desired in connection with the addition of calcium sulphite to paper as a filler, it may either be introduced into the filler before thelatter is added to the paper stock or the two may be added to the stock at the same point, or in any event the agent should be added tothe stock at or in advance of the point at which it is vigorously agitated with the filler in the presence of air. Ordinarily this will be at the beater. Similarly, in the use of the agent in connection with broke it will be most advantageous to add it at the broke beater.

While certain preferred agents and conditions have been specified in some detail it will be understood that various substitutions and modifications may be made without departing from the general principles and scope of the invention. The terms and expressions employed herein are to be regarded as terms of description and not of limitation.

By the term .water soluble salt used in the appended claims, I intend to include salts sufficiently soluble in water to permit reaction with calcium sulphate, such as barium. carbonate and barium sulphite referred to above.

What I claim is:

l The treatment of broke containing calcium sulphite as a filler and sizing material, which comprises maceration of the broke in the presence of water and a suflicient quantity of a water soluble non-reducing salt of an acid whose calcium salt is less soluble than calcium sulphate in water to substantially prevent accumulation of calcium sulphate, said salt being capable of reacting with calcium sulphate.

2. In the beating of paper pulp containing water, sizing material, and calcium sulphite as a filler, the step of adding a sufiicient quantity of a water soluble inorganic salt in which the anions are composed of three atoms of oxygen and at least one atom of sulphur and barium carbonate to substantially prevent the accumulation of calcium sulphate.

3. The treatment of paper pulp containing calcium sulphite as a filler, comprising the step of beating said pulp in the presence of water and a sufficient quantity of a water soluble non-reducing salt of an acid whose calcium salt is less soluble in water than calcium sulphate to offset the conversion of calcium sulphite to calcium sulphate in said pulp, said salt being capable of reacting with calcium sulphate.

4. The treatment of paper pulp containing calcium sulphite as a filler, comprising the step of beating said pulp in the presence of water,

sizing material, and an amount between 2 and 10% of the weight of said calcium sulphite of a water soluble non-reducing salt of an acid whose calcium salt is less soluble in water than calcium sulphate, said salt being capable of reacting with calcium sulphate to substantially prevent accumulation thereof in the pulp.

5. The treatment of paper pulp containing calcium sulphite as a filler, comprising the step of beating said pulpin the presence of water and sodium phosphate in amount between about 2 and 10% of the weight of calcium sulphite to offset conversion of calcium sulphite to calcium sulphate in said pulp.

6. The treatment of paper pulp containing calcium sulphite as a filler, which comprises beating the pulp in the presence of water and sufficient quantities of a water soluble salt of an acid whose calcium salt is less soluble in water than calcium sulphate and which is capable of reacting with calcium sulphate, and a reducing agent capable of retarding the oxidation of calcium sulphite to calcium sulphate, to offset conversion of calcium sulphite to calcium sulphate in said pulp- 7. In the beating of paper pulp containing Water, sizing material, and calcium sulphite as a filler, the addition of suflicient barium carbonate to substantially prevent accumulation of calcium sulphate.

8. In the beating of paper pulp containing water, sizing material, and calcium sulphite asv a filler, the addition of sufficient amounts of barium carbonate and a reducing agent capable .of retarding the conversion of calcium sulphite phate to offset the conversion of calcium sulphite to calcium sulphate in said pulp, said phosphate being capable of reacting with calcium sulphate.

FRANCIS G. RAWLING. 

